1. Field of the Invention
The present invention relates to short arc type discharge lamps and relates specifically to short arc type discharge lamps wherein an emitter part containing thorium oxide is provided at the cathode.
2. Description of Related Art
As short arc type discharge lamps containing mercury have a short distance between the tip ends of a pair of electrodes arranged oppositely to each other in a light emitting tube and are close to point light sources, they are conventionally used for the light source of exposure devices with a high focusing efficiency by means of a combination with an optical system. Further, short arc type discharge lamps containing xenon are used as light sources for visible light in projectors etc., and recently, they are also used as light sources for the digital cinema.
Among these short arc type discharge lamps, lamps are known which are designed to increase the electron emission characteristics by providing an emitter material at the cathode. In this regard, recently, limitations have been established for the use of thorium being an emitter material from the aspect of savings in the scarce resources of rare earth elements, and there is the demand to avoid the usage of large quantities thereof. Additionally, there is also the fact that thorium is a radioactive substance and the handling thereof is restricted by legal regulations.
With regard to these circumstances various discharge lamps have been developed which are configured such that the emitter material is contained only in the tip end part of the cathode. In JP-A-2010-33825, the configuration of the cathode of such a known short arc type discharge lamp is disclosed. FIG. 4 illustrates this conventional technique wherein FIG. 4(a) is a general view of the short arc type lamp and FIG. 4(b) shows the configuration of the cathode thereof As shown in FIG. 4(a), an anode 11 and a cathode 12 made from tungsten are arranged opposite to each other in the interior of a light emitting tube 10 of a short arc type discharge lamp 1. A light emitting substance such as mercury or xenon is enclosed in said light emitting tube 10. In this drawing, a condition is shown where the short arc type discharge lamp 1 is lighted vertically, but depending on the usage there are also lamps which are lighted horizontally. The configuration of the cathode in this lamp is shown in FIG. 4(b). The cathode 12 consists of a cathode main body part 12b made from high purity tungsten and an emitter part 12a formed integrally therewith. As to this emitter part 12a, an emitter substance such as, for example, thorium oxide has been introduced into tungsten. When thorium is used for the emitter substance in this kind of lamps, the thorium oxide contained in the thoriated tungsten of the tip end part of the cathode is reduced to thorium atoms by means of the development of a high temperature at the cathode surface, diffuses at the outer surface of the cathode, and migrates to the tip end side where the temperature is high. By means of this, the work function can be decreased and the electron emission characteristics can be improved.
In the above mentioned example of the prior art, however, the emitter substance contributing to the improvement of the electron emission characteristics is actually limited to the emitter substance contained within a very shallow region from the outer surface of the cathode tip end. Originally, the emitter substance evaporates at the outer surface of the cathode tip end because of the heat and is consumed, but it is expected that emitter substance will be supplied because of the concentration diffusion from the interior of the cathode. But there is the phenomenon that the supply because of the concentration diffusion from the interior of the cathode having a lower temperature becomes insufficient as compared to the amount being consumed at the outer surface where the temperature is highest, and that the supplied quantity cannot reach the consumed quantity. Thus, even if a large quantity of the emitter substance is contained in the interior of the cathode, the phenomenon arises that there is a shortage of the emitter substance at the surface of the cathode. Therefore, with the above mentioned known technique there is the problem that although an emitter substance is contained in the cathode tip end, this emitter substance is not utilized sufficiently, and when the emitter material at the cathode tip end surface is depleted, the electron emission characteristics decrease and flicker occurs.